1. Field of the Invention:
The invention disclosed herein relates to a raster display generating system having means for converting calligraphic symbology information into raster scanned symbology, and more particularly, to such a system wherein symbols are stroked into a raster image buffer for later display on a raster scanned matrix display in raster scanned format.
2. Prior Art:
There are two well-known methods whereby images are formulated on a display device such as a cathode ray tube (CRT). They are calligraphic or stroke image generation and raster scanned image generation. Calligraphic image generation is analogous to writing with a pen. The pen is first positioned at the point where the symbol is to be drawn and then the symbol is stroked out. The pen is positioned for the next symbol and then that symbol is stroked out and so on. Raster scanned image generation is somewhat more complex. The CRT electron beam continuously scans the face of the CRT from left to right, top to bottom (or in some other predefined directions). The beam starts at the upper left hand corner of the display and sweeps to the right; when it gets to the extreme right edge of the display, the beam snaps back to the left side and begins sweeping the next raster display line just below the previous line. It continues to do this until it has swept the entire face of the display device, ending at the bottom right hand corner of the display. At this point the beam snaps back to the top left of the display and begins the process over again. In order for the electron beam to display a symbol on the display, the beam must be turned on and off, that is, blanked and unblanked, in a programmed manner such that a symbol image is formed at the desired point on the display. Since the electron beam does not stop, but instead continues to sweep repetitively across the CRT's face, the symbol generator must know, or predict, where the beam is in order to formulate the image. At a given point on a selected raster line, the beam must be unblanked and then blanked according to a program to generate the top of the symbol. Again on the next succeeding raster line, the beam must be unblanked and blanked to generate the next portion of the symbol. This process continues on to the bottom of the symbol; i.e. the last raster line that the symbol appears. Complications set in when there are a multiplicity of symbols of various shapes and which move about the display according to the functions they represent. Hence, it is more difficult to generate a raster image than to generate a calligraphic image. Nevertheless, a raster display device dissipates less power and is smaller and cheaper than a comparable calligraphic display device. This is important in an aircraft cockpit environment where instrument panel space is at a premium and where the cockpit environment must be cooled. Furthermore, most image sensors for aircraft cockpit applications are presented in a raster format because of cost, size, and complexity. The use of a raster display system improves compatibility and removes the complexity from the display unit in the cockpit to the display generator unit in the equipment bay of the aircraft. Nevertheless calligraphic displays have predominated in aircraft systems until recently because of the display brightness and the overwhelming display generator complexity of raster systems. Improvements, however, have occurred in both of these areas to the point where raster imagery is now becoming the major type of aircraft display system.
There are two methods for generating raster imagery: (1) real time, hardware generation and (2) computed imagery that is stored in a refresh memory. The display generator complexity of the first depends upon the type of imagery displayed. If there are many symbols of various shapes and sizes which must translate over the display face, and if symbols are required to rotate and roll about the display face, the display generator will contain a large amount of hardware. If the display is a text format, then the display generator will be rather simple. The display generator of the second method is much more versatile and in the past included a computer that computed the symbol's shape, size and position, storing them in a refresh memory. The refresh memory would then be scanned in synchronism with the sweep of the electron beam across the CRT face, and according to the data within the refresh memory, the beam would be modulated thereby generating the images. For a complex display, this involved a very large computer, but any symbol could be generated and displayed. Until the advent of integrated circuit random-access-memory (RAM) devices, the physical size of the memory was quite large. This type of system, therefore, was not compatible for aircraft cockpit displays.
It is desirable, therefore, to provide a system and a method for generating a complex raster display including means for stroking symbology into a refresh memory using calligraphic symbol generation techniques and ultimately to provide such symbology in raster scanned format to a raster scanned matrix display for presentation.